What happens when a Hidden Unit card is placed on Rockfall Path and someone tries to activate it?

Ruling: Hiding a unit card facedown on Rockfall Path is legal, but when you attempt to play that unit, it is an illegal activation. The play is rewound, the card must remain hidden and cannot be played, and it gets trashed when you lose control of the battlefield.

Sequence:

• Card is legally hidden facedown on Rockfall Path
• When attempting to play the unit, the activation is illegal
• The play is rewound to before the illegal activation
• Card remains hidden and cannot be played
• Card is trashed when control of the battlefield is lost

Nuances:

• If a spell or ability instructs you to play a unit to Rockfall Path, you can shortcut the instruction to simply not execute (the spell/ability resolves to no effect)
• Such spells/abilities can still be played for purposes like triggering Ravenbloom Student or Darius Trifarian, they just resolve to no effect